Many types of inertial sensors are known. Angular velocity sensors incorporating a MEMS type ring are known and such examples can be seen in, for example, GB2322196. In such angular velocity sensors a vibrating planar ring or hoop-like structure is disclosed. The ring-like structure is suspended in space by a suitable support mount for detecting turning rate, linear acceleration and angular acceleration. Turning rate is sensed by detecting vibrations coupled by Coriolis forces, whereas linear acceleration and angular acceleration are sensed by lateral, vertical and rocking movement of the entire ring or hoop-like structure within its mount.
These sensors are required to accurately measure signals derived from stimuli which are also controlled by the sensor. In the case of coriolis gyroscopes, the linear velocity is setup and controlled by a transducers and the rate response is also measured by a ‘similar’ transducer.
Typically the MEMS ring type structures are formed from crystalline silicon and incorporate transducers formed from Lead Zirconate Titanate (PZT) materials. PZT materials have significant benefits in terms of signal level (and therefore signal to noise ratio levels) when used as the transducer technology in low cost MEMS sensor applications. However PZT suffers from fundamental ageing and temperature drifts compared to other transducers technologies (inductive, capacitive). Gain variations due to temperature effects may cause the rate sensor scalefactor to vary and this error can be compensated by using an independent temperature sensor but ageing is normally a limiting factor. In addition the phase accuracy of the transducers is a key error in determining the bias (Zero offset) of the overall rate sensor due to the presence of very large quadrature bias signals which are at 90 degrees (electrical) phase to the required rate signal and typically 3 orders of magnitude larger. The phase is affected by the PZT material characteristics, particularly capacitance and is also subject to large temperature and ageing related drifts. The result of all these issues is that the use of PZT transducers in MEMS sensors is limited and PZT is generally only used in low performance applications or where stability over a short timescale is required, for example in GPS aided navigation systems.